During WW2, America bombed a lot of civilians both in the Empire of Japan- including long before Hiroshima as well as in Germany.
And the axis did the same kind of thing too.
It was just part of the war- the fact that new technology- the A-bomb- was used isn't particularly shameful. The Axis used the equally new V2 missile technology to attack the Allies civilians during the same conflict. Has Merkel apologized for that?
If the UK had the bomb, they would have used it.
If France had the bomb, they would have used it.
If POLAND, had the bomb, they would have used it.
If the Netherlands had the bomb, they would have used it.
The fact that we are the ones that did use it, is a testimony to our technical and scientific and economic prowess.
Not a sign of a moral failing.